1. Field of the invention
The present invention relates to an information recording medium which is advantageously employed for recording or reproducing optical information by means of a laser beam of high energy density and a method of optically recording information using said information recording medium.
2. Description of prior art
Information recording media utilizing a beam of high energy density such as a laser beam have been developed in recent years and are now put into practical use. Such recording medium is generally called "optical disc", and the practical applications thereof have been found, for example, as a video disc and an audio disc as well as a disc memory for a large-capacity computer and a large-capacity static image file. Among these media, a compact disc (referred to as "CD") is now widely used as an audio disc for reproducing a music or the like.
The compact disc (CD) for reproduction of information only is generally rotated at a fixed linear speed of 1.2 to 1.4 m/sec., according to the predetermined CD standard to read out (reproduce) the recorded information therefrom, and CD is required to allow a long recording time (up to 74 min.) using a signal-recording area within the region between inner diameter of 45 mm and outer diameter of 116 mm under the conditions of pit width of 0.8 .mu.m and track pitch of 1.6 .mu.m. The conventional audio compact disc is used only for reproduction. Such compact disc has been beforehand provided with pits on the substrate (accordingly not provided with an independent recording layer), so that the compact disc has such problems that information cannot be recorded or edited thereon on the users' side. Accordingly, development of a compact disc of DRAW (Direct Read After Write) type (i.e., disc capable of being written) is required, and some compact discs of DRAW type have been put into practical use. The above requirement is also addressed to discs for memorizing various documents, data, static image files, etc., discs of DRAW type for CD-ROM (Read Only Memory) or CD-I (Interactive).
The information recording medium (optical disc) of DRAW type has been already known, and the information recording medium basically comprises a disc-shaped transparent substrate made of plastic or glass material and a recording layer made of metal or semi-metal such as Bi, Sn, In and Te provided on the substrate. The recording layer may be made of other material such as dye. Writing of information on the optical disc can be conducted, for example, by irradiating the medium with a laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the disc absorbs energy of the beam and rise of temperature locally occurs, and as a result, chemical and physical change is caused to alter optical characteristics of the recording layer in the irradiated area, whereby recording of information can be made. Reading of information from the optical disc is also conducted by irradiating the disc with a laser beam. The recorded information can be reproduced by detecting a reflected light or a transmitted light corresponding to the alteration of the optical characteristics of the recording layer.
In the known information recording media of DRAW type, the information cannot be recorded unless the medium is rotated at a high linear speed. Therefore, it is required to have wide spaces between adjoining pits for the recording, and hence it is almost impossible to record information thereon according to the CD system in which high density recording is made at a low linear speed. Even if the conventional DRAW type recording medium would record information thereon, the recorded information could not be read out therefrom by means of a commercially available CD player, because the commercially available CD player can read only an optical disc having a high reflectance for a light for reproduction such as a laser beam.
As described above, writing of information on the optical disc or reading of information from the optical disc is generally conducted by irradiating a predetermined portion of the disc surface with a laser beam. Recently, as a large amount of information is desired to be recorded on one optical disc, it has been proposed to increase density of the units for record of information in the form of such as pits in recording area of the disc. However, the increase of density of the recording units causes difficulty in precisely irradiating the pre-determined portion of the disc with a laser beam, so that optical discs in recent years generally have on their surfaces a tracking guide in the form of spiral concave groove (i.e., pre-groove) as shown in FIG. 1 of attached drawings for precisely guiding the laser beam to the pre-determined irradiation area (generally referred to as "tracking"). The tracking guide of such form is generally arranged on optical discs of CD format.
In the case that the concave groove is provided on an optical disc, the difference between the amount of a reflected light in the area where pits are formed and that of a reflected light in the area where pits are not formed can be made larger, so that such groove is of great value for known optical discs (especially for optical discs according to International Organization for Standardization (ISO)), and usually provided on those discs. For reproducing information from an optical disc of CD format, a commercially available CD player is required, and hence the optical disc should show a high reflectance during the reproducing procedure. However, in the above-mentioned concave groove, the irradiated laser beam is apt to be diffracted, and hence a required large amount of the reflected light is hardly received by a receptor of the CD player. Accordingly, it is difficult to obtain a sufficiently high reflectance using a tracking guide in the form of the above-mentioned conventional concave groove.